A permanent or semi-permanent horizontal movement of unusually cold or warm surface water of the oceans, to a depth of about 100 m. Ocean currents are generated by the thrust of the wind, the spin of the Earth, and the Moon's gravity, and are also affected by variations in water temperature and density, and by the Coriolis force. These currents are an important factor in the redistribution of heat between the tropics and the polar regions; see thermohaline circulation. The Southern Ocean, for example, is ‘a crucial cog in the global heat engine’ (anon. (2002) Australian Arct. Mag.). Saetra et al. (2008) Tellus A 60, 1 believe that an increase in the sea-surface temperatures of 1–2 °C in the North Atlantic current may intensify cyclones. See also Ridgway and Dunn (2007) Geophys. Res. Letts 34, 13.

Cold currents can modify temperatures up to 100 km inland. As tropical air streams move over these currents, advection fog forms over the sea, stripping the air streams of most of their moisture; onshore winds are therefore dry; see T. T. Warner (2004). Conversely, warm currents originate in tropical waters, bringing unusually warm conditions to coastal areas in higher latitudes. The North Atlantic Drift, for example, creates a difference of around 4 °C between the winters of coastal Canada and coastal western Europe of the same latitude (Bryden in G. Siedler and J. Church, eds 1999).